In the applications of radio frequency (RF), a higher cut-off frequency is always required. Although an RFCMOS can achieve a relatively high frequency by using an advanced process, it still can not fully meet the requirements of radio frequency, for example, it is difficult to achieve a cut-off frequency above 40 GHz. Besides, the cost for research and development of advanced process is rather high.
It is known that compound semiconductors can be used to manufacture devices with very high cut-off frequencies. However, the application of compound semiconductors is limited because of their disadvantages such as high cost, small size, and poisonousness.
In recent years, silicon-germanium heterojunction bipolar transistor (SiGe HBT) has become a good choice for super high frequency devices for the following reasons: firstly, due to the difference between the energy bands of silicon-germanium (SiGe) and silicon (Si), the carrier injection efficiency of the emitter region is increased, and the current amplification performance of the device is improved; secondly, the heavily doped SiGe base region can reduce the base resistance and increase the cut-off frequency; thirdly, the SiGe process is basically compatible with the silicon process and is not high in cost. Therefore, SiGe HBT has become a main force of super high frequency devices.
A PNP bipolar transistor is an important device of the silicon-germanium BiCMOS (SiGe BiCMOS) process other than a SiGe NPN HBT. In a conventional SiGe BiCMOS process, a PNP device is designed to have a lateral structure for easy pick up of the collector region formed in the P-well. However, the greatest weakness of a lateral PNP transistor is that the width of the base region is large and the current-gain is small.